This invention relates in general to signal switches and in particular, to a fast transmission gate switch, particularly useful for switching digital logic signals.
With the advent of very large-scale integrated circuits, the size of devices manufactured has been shrinking and the speed of the devices has continually increased. Most of the efforts, however, have been directed to designs where many circuits and the connections between them are made in the same medium such as a silicon chip. Since all the circuit components being fabricated are done in the same medium, devices may be made smaller by improved techniques such as improved lithography. By reducing the sizes of the devices and the lengths of the connections between them, the speed of the device is increased due to a decrease in the inductances, resistances and capacitances of individual devices and of the connections between them.
Board level designs have not kept pace with the above-described development in very large-scale integrated circuits. Printed circuit board designs frequently have large capacitances which slow down signal propagation. Thus if two high speed logic chips are connected through a slower device, the overall speed of the system is determined by the slowest component, namely, the slow connecting device. Passive switching devices such as transmission gates have been used in printed circuit board level designs for switching signals between digital logic devices. The slow speed of this type of switches determines the speed of signal transmission even though the two logic devices may operate at much higher speeds. It is therefore desirable to provide a stand-alone switching device which is much faster than the conventional passive transmission gate.
In one type of improved switching circuits that is frequently used, instead of a passive switch, an active device such as a logic buffer is used. While a buffer used as a switch causes a delay that is less than that caused by the conventional passive transmission gate switch, the delay caused by such buffers may nevertheless be excessive and undesirable for some high speed applications. It is thus desirable to provide a high speed switch that causes less delay than the above described switches.
The switching device of this invention has two input/output ports for passing or blocking signals between the two ports of the device. The device comprises a transistor having a first and a second terminal and a control terminal. The first and second terminals are connected between the two ports. The transistor thereby passes signals between the two ports when the transistor is turned on and blocks the passage of signals between the two ports when the transistor is turned off. The resistance between the first and second terminals is less than about 10 ohms when the transistor is turned on. The device further comprises means for applying a switching signal to the control terminal of the transistor for turning it on or off. In the preferred embodiment, the capacitance between the first or second terminal and a reference potential is less than about 50 pF when the transistor is turned on.